The advent of autonomous regional satellite based navigation systems has enabled some countries to cover their territorial footprint and the footprint of their surrounding areas. The purpose of a regional satellite based navigation system, such as a global navigation satellite system (GNSS), is to cater to the needs of specific users, for example, the military, for military applications using a precision service (PS) and civilian users, for example, for civilian applications using a standard positioning service (SPS). For example, India is planning to deploy an autonomous regional satellite based navigation system, namely, the Indian regional navigational satellite system (IRNSS), for surveying, telecommunication, transportation, identifying disaster locations and public safety, etc. The IRNSS will deploy a satellite constellation comprising seven satellites, of which three satellites are in geostationary orbits and four satellites are in geosynchronous orbits.
A number of issues need to be addressed in the design of a satellite navigation system for ensuring the efficiency and robustness of the satellite navigation system, for example, sensitivity improvements, jamming margins, robustness towards spoofing, multipath related improvements, time to first fix (TTFF), etc. One design parameter that needs to be optimized is the TTFF parameter, which is a measure of time needed by a satellite navigation receiver to acquire satellite signals and navigation data, and calculate a position solution, referred to as a “fix”. The TTFF parameter directly influences the efficiency of position tracking by the satellite navigation receiver.
The TTFF parameter is an important receiver specification parameter that serves as a yardstick for comparing satellite navigation receivers from different manufacturers. In order to process a navigation signal emanating from a satellite, a global positioning system (GPS) L1 frequency receiver first establishes a lock on code and carrier frequency. Subsequently, in the lock condition, navigation data from the satellite is demodulated. Conventional satellite navigation receivers require a minimum of four satellites to compute the user navigation solution based on navigation measurements, for example, pseudorange measurements, delta range measurements, etc., and satellite state vectors, for example, the position of the satellite position, the velocity of the satellite, etc. For an optimal TTFF performance, it is necessary that the time taken for computing navigation measurements and subsequent navigation data collection is minimal. Typically, methods for reducing TTFF have focused on reducing the time required to acquire and lock the navigation signal, assisting the satellite navigation receiver with navigation data on a separate satellite link, etc. However, these methods are generally expensive in terms of deployment costs, complexity of the satellite navigation receiver, etc. Furthermore, conventional methods for reducing TTFF have often been ineffective in processing navigation signals for reduction of line of sight (LOS) TTFF.
A typical global navigation satellite system (GNSS) signal may be characterized by the following equation:s(t)=c(t)*[r(t)⊕d(t)]where the parameter s(t) refers to an output GNSS signal at a time instant t, the parameter r(t) refers to a ranging code at the time instant t, the parameter c(t) refers to a frequency of operation at the time instant t, and the parameter d(t) refers to the navigation data transmitted by each satellite.
The navigation data transmitted by each satellite can be grouped into ephemeris data and almanac data. The ephemeris data comprises precise clock and Keplerian parameters, which are typically updated once every two hours. Typically, the ephemeris data or ephemerides are transmitted periodically once every two hours. The almanac data provides a coarse estimate of a satellite orbit, which is primarily used for satellite visibility computations. The almanac data also comprises ionosphere delay estimation coefficient for single frequency users, for example, GPS L1 users. The almanac data typically changes once in a day. The satellite state vectors of a satellite computed using the ephemeris data are used for estimation of a user position and velocity.
The satellite navigation receivers of conventional satellite navigation systems have generally been constrained by the amount of time taken for collecting the ephemeris data and the almanac data that constitute the navigation data. The delay in collecting the navigation data translates to multiple delays, for example, delays in computing satellite visibility, delays in estimation of ionosphere delay estimation coefficients, delays in cross-correlation detection based on the range estimated using the almanac data and an integrity check specified by the federal aviation administration (FAA) for beta-3 civil aviation receivers, etc. Conventional satellite navigation receivers take a relatively long time, for example, about 12.5 minutes to collect the almanac data for a single frequency user. This delays the estimation of ionosphere error, which is an important parameter for estimation of the position of the satellite.
The typical time taken by a user in open sky conditions to collect ephemeris data and almanac data from the global positioning system (GPS) and the global navigation satellite system (GLONASS) is recorded. The ephemeris data collection time in the GLONASS is, for example, about 30 seconds, and the ephemeris data collection time in a GPS is, for example, about 30 seconds. The almanac data collection time in a GLONASS is, for example, about 150 seconds (s), while the almanac data collection time in a GPS is, for example, about 750 seconds.
Furthermore, a conventional satellite navigation system such as the global positioning system (GPS) transmits the navigation (NAV) data, for example, as “sub-frames”, while the GLONASS transmits the NAV data, for example, as “strings”. In GPS, Keplerian parameters are transmitted as a part of the navigation data while in GLONASS, the absolute state vectors of a satellite are transmitted. Existing GPS based systems employ an L1 sub-frame structure, with the first three sub-frames constituting the ephemeris data and the last two sub-frames dedicated to the almanac data. Each sub-frame contains 10 words and each word has 24 navigation data bits and 6 parity bits. The use of 6 parity bits per word translates to 60 parity bits per sub-frame, effectively constraining the data bandwidth and delaying the time to first fix. The data bits are transmitted at 50 bits per second (bps). Therefore, one complete sub-frame is transmitted in 30 seconds. For every 24 bits, six redundant bits are transmitted. This constrains the time taken for the collection of the ephemeris data and delays the TTFF.
Furthermore, in each sub-frame, existing words and bits, for example, telemetry (TLM) data, hand over word (HOW) data, sub-frame identifier, etc., need to be transmitted. The almanac data is transmitted in two sub-frames, for example, sub-frames 4 and 5. In addition, the almanac data comprises ionosphere correction terms and coordinated universal time (UTC) parameters. Further, in case of current almanac transmission methods deployed in GPS, at any given instant of time, all satellites transmit the same information as part of sub-frames 4 and 5. The sub-frames 4 and 5 transmit almanac data for all the 25 pages with each almanac page comprising the almanac data of a particular satellite. Furthermore, with the current scheme of almanac data transmission, as for example in GPS based systems, it takes about 168 seconds for a seven-satellite satellite navigation system to collect almanac data. This delays the ionosphere error computation and thus delays accurate positioning in a satellite navigation receiver. Moreover, parameters such as UTC parameters compound the delay and bandwidth overhead since the UTC parameters need not be transmitted very frequently for computation of the user's position.
A navigation (NAV) data structure of the Galileo GNSS adopts a sub-frame architecture similar to that of a GPS based system. The navigation data structure uses a 12 sub-frame structure with each sub-frame comprising a series of pages. Each page comprises a synchronization pattern, and navigation data symbols. Each navigation data symbol comprises a navigation data word and tail bits. The navigation data word comprises a 24-bit cyclic redundancy check (CRC) code. Further, as in the case of other modern GNSS systems, the Galileo adopts a half rate forward error correction (FEC) encoding. For the Galileo GNSS, the integrity bits are added to a packet of navigation data. However, Galileo uses a navigation data structure with a larger number of sub-frames and imposes constraints in terms of memory requirements and an increased amount of time required for transmitting the complete navigation data structure.
The GPS L5 satellite navigation system is a global navigation satellite system that employs navigation data transmission based on the transmission of text messages at a predefined rate. Each text message is identified based on a message identifier (ID). GPS L5 uses a half rate forward error correction (FEC) encoding scheme with a baud rate of 100 symbols per second (sps). The signal transmitted from an L5 satellite is at a power level of, for example, about −157 decibel-watt (dBW). The GPS L5 system allows variation of frequency of text message transmission. However, the GPS L5 system continues to employ a five sub-frame structure and needs about 30 seconds for complete transmission of all the sub-frames of the navigation data structure.
Furthermore, the transmission of almanac data in a conventional GNSS, for example, a GPS based system comprises transmission of the same almanac data by all satellites in a constellation. For example, in a seven-satellite constellation employed by the Indian regional navigational satellite system (IRNSS), each satellite transmits the same almanac data at each time instant over a satellite channel. This increases the time overhead in almanac transmission and increases the almanac data collection time at the satellite frequency receiver, thereby delaying ionosphere estimation at the satellite navigation receiver. For example, in the IRNSS, seven almanac pages need to be transmitted as a part of the third sub-frame. On using an almanac transmission scheme typically used in GPS, it takes about 168 seconds to completely transmit the almanac data.
Hence, there is a long felt but unresolved need for a method and system that reduces the time to first fix in a satellite navigation receiver by enabling faster access to the ephemeris data and the almanac data.